OBJECTIVES. To evaluate the prevalence of human epidermal growth factor receptor 2 (HER2) gene overexpression in breast cancer patients encountered in Hong Kong and the concordance of HER2 findings from primary immunohistochemistry assays and confirmatory in-situ hybridisation assays.

DESIGN. Retrospective study.

SETTING. Department of Clinical Oncology in a public hospital in Hong Kong.

PATIENTS. All patient referrals between July 2006 and June 2007 with newly diagnosed invasive breast cancer (for prevalence evaluation), and all patients treated at our unit with confirmatory in-situ hybridisation tests performed within the study period (for concordance evaluation).

MAIN OUTCOME MEASURES. Prevalence of Mycoplasma genitalium and Ureaplasma urealyticum among symptomatic male patients with non-gonococcal urethritis and asymptomatic patients without non-gonococcal urethritis.

RESULTS. There were 272 consecutive breast cancer patients eligible for prevalence evaluation. The distribution for immunohistochemistry staining in 249 cases for scores 0, 1+, 2+, and 3+ were 99 (40%), 40 (16%), 58 (23%), and 52 (21%) respectively. In the remaining 23 patients, four and 19 breast cancers were unscored and reported by immunohistochemistry to be HER2-positive and -negative, respectively. The overall HER2 overexpression rate (3+ or reported as positive) was 21%. HER2 overexpression was associated with grade 3 histology (P<0.001) and negative hormonal receptor status (P<0.001). However, it was not associated with age (P=0.525), T-classification (P=0.740), N-classification (P=0.691), nor group stages (P=0.433). Of the 37 patients with confirmatory in-situ hybridisation tests performed, 10 (71%) of 14 with immunohistochemistry staining of 3+ and 1 (4%) of 23 with immunohistochemistry staining of 2+ were found to have HER2 gene amplification.

CONCLUSIONS. More than 25% of HER2 overexpression identified by immunohistochemistry assays in this Hong Kong cohort could not be verified by confirmatory in-situ hybridisation assays. Compliance with the latest guidelines for HER2 testing should improve the future accuracy and concordance.

Key words: Breast neoplasms; Gene amplification; Immunohistochemistry; In situ hybridization, fluorescence; Receptor, epidermal growth factor

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